In an oscillation circuit such as a 3× (3rd) overtone oscillation circuit or an oscillator used as a voltage controlled crystal oscillator (VCXO), a coil (or an inductor) called an “extension coil” is typically connected in series in order to secure a variable frequency bandwidth. A coil is a discrete part, and its size is roughly determined by an inductance required for the coil, which hinders miniaturization of the entire oscillation circuit and oscillator.
FIG. 1 is a circuit diagram illustrating an example of an oscillator. The oscillator 1 illustrated in FIG. 1 includes a 3rd overtone oscillation circuit as an example of an oscillation circuit, a crystal unit 2, and an extension coil 3. The 3rd overtone oscillation circuit includes resistors R1 to R3, an inductor L1, capacitors C1 to C6, a variable capacitor C7, and a transistor Tr, all of which are connected as illustrated in FIG. 1. A power source voltage is applied to a terminal T4, and an output signal of the oscillator 1 is output from a terminal 5. An inductance value of the extension coil 3 is, for example, several μH or so.
FIGS. 2A to 2C illustrate an example of a conventional crystal unit 200, FIG. 2A being a plan view of the crystal unit 200, FIG. 2B being a sectional view of the crystal unit 200, which is taken along a dashed line B-B in FIG. 2A, and FIG. 2C being a sectional view of the crystal unit 200, which is taken along a dashed line C-C in FIG. 2A. In FIGS. 2A to 2C, the crystal unit 200 includes a package 21 made of, e.g., ceramics or the like, a pair of excitation electrodes 22, a crystal substrate 23, a conductive adhesive 24, four electrodes 25-1 to 25-4, and wirings 26-1 and 26-2. The pair of excitation electrodes 22 is disposed on both surfaces of the crystal substrate 23 with the crystal substrate 23 sandwiched therebetween. For example, the lower excitation electrode 22 is electrically connected to the electrode 25-1 by the conductive adhesive 24 via the wiring 26-1. In the sectional view of FIG. 2B, for the purpose of facilitating the understanding of the electrical connection, the wiring 26-1 and the electrode 25-1 (and the electrode 25-2), which are originally invisible, are indicated by a dashed line for the sake of convenience. The upper excitation electrode 22 is electrically connected to the electrode 25-2 by the conductive adhesive 24 via the wiring 26-2. In the sectional view of FIG. 2C, for the purpose of facilitating the understanding of the electrical connection, the wiring 26-2 and the electrode 25-2 (and the electrode 25-1), which are originally invisible, are indicated by a dashed line for the sake of convenience. The wiring 26-1 includes a via connected to the electrode 25-1 through the package 21. Similarly, the wiring 26-2 includes a via connected to the electrode 25-2 through the package 21. In this case, the electrodes 25-3 and 25-4 are not electrically connected to the excitation electrode 22 and can be used as, e.g., pads when the package 21 is installed. However, the electrodes 25-3 and 25-4 may be electrically connected to the excitation electrode 22.
The size of the package 21 on the plan view is, for example, 3.2 mm×2.5 mm, as illustrated in FIG. 2A, and the height of the package 21 on the side view is, for example, 0.5 mm to 0.7 mm, as illustrated in FIGS. 2B and 2C. When a chip coil, which is a discrete part, is used for the extension coil 3 having the inductance of several μH or so as mentioned above, the size of the chip coil is, for example, 1.0 mm×0.5 mm×0.5 mm. However, when the chip coil is connected to the crystal unit 200 as illustrated in FIGS. 2A to 2C, the size of the portion surrounded by a dashed line in FIG. 1 becomes relatively large, which makes it difficult to miniaturize the oscillator 1.
Therefore, it is conceivable to use an extension coil, which is not a discrete part, for the crystal unit. However, when this chip coil is disposed in the side of the crystal unit within the package, the occupation area of the package increases, which makes it difficult to miniaturize the oscillator 1. Meanwhile, when this chip coil is disposed above or below the crystal unit within the package, although the mounting area corresponding to the chip coil may be reduced as compared to the case where the chip coil is disposed in the side of the crystal unit, the height of the package increases, which also makes it difficult to miniaturize the oscillator 1. In addition, when the chip coil is made thin, it is difficult to obtain the inductance of, e.g., several μH or so required for the extension coil.
In this way, it is difficult to miniaturize the crystal unit with a built-in a coil where an extension coil is built-in since the size of the extension coil is roughly determined by the inductance required for the extension coil.
It is difficult to install a coil without compromising the size of the crystal unit with a built-in coil.
The followings are a reference documents.    [Document 1] Japanese Laid-Open Patent Publication No. 2014-023015,    [Document 2] Japanese Laid-Open Patent Publication No. 2013-258571, and    [Document 3] Japanese Laid-Open Patent Publication No. 02-226905.